Gee was devised by Robert Dippy as a short-range blind landing system to improve safety during night operations, but during development by the Telecommunications Research Establishment (TRE) at Swanage, the range was found to be far better than expected and it developed into a long-range, general navigation system. For large, fixed targets, such as the cities that were attacked at night, Gee offered enough accuracy to be used as an aiming reference without the need to use a bombsight or other external reference. Jamming reduced its usefulness as a bombing aid, but it remained in use as a navigational aid in the UK area throughout the war.

Gee remained an important part of the Royal Air Force's suite of navigation systems in the postwar era, and was featured on aircraft such as the English Electric Canberra and the V-bomber fleet. It also had civilian use, and several new Gee chains were set up to support military and civil aviation across Europe. The system started to be shut down in the late 1960s, with the last station going off the air in 1970. Gee also inspired the original LORAN ("Loran-A") system

The basic idea of radio hyperbolic navigation was well known in the 1930s, but the equipment needed to build it was not widely available at the time. The main problem involved the accurate determination of the difference in timing of two closely spaced signals, differences in milli- and microseconds.[1]

During the 1930s, the development of radar demanded devices that could accurately measure these sorts of signal timings. In the case of Chain Home, transmission aerials sent out signals, and any reflections from distant targets were received on separate aerials. An oscilloscope (or oscillograph as it was known in the UK)[1] was used to measure the time between transmission and reception. The transmitter triggered a time base generator that started a "trace" moving quickly along the oscilloscope display. Any received signals caused the beam to deflect downward, forming a blip. The distance that the trace had moved from the left side of the display could be measured to accurately calculate the difference in time between sending and receiving, which, in turn, could be used to calculate the slant range to the target.[1]

Radar can also be used as a navigation system. If two stations are able to communicate, they could compare their measurements of the distance to a target, and use basic triangulation to determine the location. This calculation could then be sent to the aircraft by radio. This is a fairly manpower-intensive operation, and while it was used by both the British and Germans during the war, the workload meant it could generally only be used to guide single aircraft.[2][3]

In October 1937, Robert (Bob) J. Dippy, working at Robert Watson-Watt's radar laboratory at RAF Bawdsey, proposed using two synchronized transmitters as the basis for a blind landing system. He envisaged two transmitting antennae positioned about 10 miles apart on either side of a runway. A transmitter midway between the two antennae would send a common signal over transmission lines to the two antennas, which ensured that both antennas would broadcast the signal at the same instant.[1]

A receiver in the aircraft would tune in these signals and send them to an A-scope-type display, like those used by Chain Home. If the aircraft were properly lined up with the runway, both signals would be received at the same instant, and thus be drawn at the same point on the display. If the aircraft were located to one side or the other, one of the signals would be received before the other, forming two distinct peaks on the display. By determining which signal was being received first, pilots would know that they were closer to that antenna, and would be able to recapture the proper direction by turning away from it.[1][b]

Watt liked the idea, but at the time, a pressing need for the system was not apparent.[1] At the time, the RAF relied on daylight bombing by tight formations of heavily defended bombers as its primary attack force, so night landings were not a major concern. Landing aids would be useful, but radar work was the more urgent need.[1]

The RAF's bombing campaign plans quickly went awry, especially after the Air Battle of the Heligoland Bight in 1939. Contrary to prewar thinking, the bombers proved extremely vulnerable to both ground fire and attacking fighters. After some discussion, the best course of action was decided to be to return to night bombing, which had been the primary concept earlier in the 1930s.

This raised the need for better landing aids, and for night navigation aids in general. Dippy refined his system for this purpose, and formally presented a new proposal on 24 June 1940.[1][4][5] The original design used two transmitters to define a single line in space, down the runway centerline. In his new concept, charts would be produced illustrating not only the line of zero-difference, where the blips were superimposed like the landing system, but also a line where the pulses were received 1 μs apart, and another for 2 μs, etc. The result would be a series of lines arranged at right angles to the line between the two stations.[5]

A single pair of such transmitters would allow the aircraft to determine on which line they were, but not their location along it. For this purpose, a second set of lines from a separate station would be required. Ideally, these lines would be at right angles to the first, producing a two-dimensional grid that could be printed on navigational charts. To ease deployment, Dippy noted that the station in the centre could be used as one side of both pairs of transmitters if they were arranged like an L. Measuring the time delays of the two outlier stations relative to the centre, and then looking up those numbers on a chart, an aircraft could determine its position in space, taking a fix. The gridded lines on the charts gave the systems its name, "Gee" for the "G" in "Grid".[5]

As the system was now intended to offer navigation over a much wider area, the transmitters of a single station would have to be located further apart to produce the required accuracy and coverage. The single-transmitter, multiple-antenna solution of the original proposal was no longer appropriate, especially given that the stations would be located far apart and wiring to a common point would be difficult and expensive. Instead, Dippy described a new system using individual transmitters at each of the stations. One of the stations would periodically send out its signal based on a timer. The other stations would be equipped with receivers listening for the signal arriving from the control station. When they received the signal, they would send out their own broadcasts. This would keep all the stations in synchronization, without the need for a wire between them. Dippy suggested building stations with a central "master" and three "slaves" about 80 miles (130 km) away and arranged roughly 120 degrees apart, forming a large "Y" layout. A collection of such stations was known as a chain.[6][5]

The system was expected to operate over ranges around 100 miles (160 km), based on the widely held belief within the UK radio engineering establishment that the 30-MHz shortwave signals would have relatively short range. With this sort of range, the system would be very useful as an aid for short-range navigation to the airport, as well as helping bombers form up after launch at an arranged location. Additionally, after flying to their cruising altitude, the bombers could use the Gee fix to calculate the winds aloft, allowing them to more accurately calculate dead reckoning fixes after the aircraft passed out of Gee range.[6]

Experimental systems were being set up in June 1940. By July, to everyone's delight, the system clearly was usable to at least 300 miles (480 km) at altitudes of 10,000 feet (3.0 km). On 19 October, a fix was made at 110 miles at only 5,000 feet.[4]

The discovery of Gee's extended range arrived at a pivotal point in the RAF's bombing campaign. Having originally relied on day bombing, the RAF had not invested a tremendous amount of effort on the navigation skills needed for night flying. When the night-bombing offensive started, the Germans were found to have developed a series of radio aids for this, notably the X-Gerät system, but the RAF initially pooh-poohed this approach, claiming it only demonstrated the superiority of the RAF's training.

By late 1940 a number of reports were trickling back from observers in the field, who were noting bombers did not appear to be bombing their targets. In one instance, bombs reportedly fell over 50 mi (80 km) from their target. For some time, these results were dismissed, but calls for an official enquiry led to the Butt report, which demonstrated only 5% of the bombs sent out on a mission landed within 5 mi (8.0 km) of their targets. With these statistics, any sort of strategic campaign based on attacks against factories and similar targets was hopeless. This led to Frederick Lindemann's notorious "dehousing" paper, which called for the bomber efforts to be used against the houses of the German citizens to break their ability to work and will to resist. This became official policy of the RAF in 1942.

While the debate raged, Bomber Command dramatically lowered their sortie rate, awaiting the rebuilding of the force with the newly arriving 4-engine "heavies" such as the Handley Page Halifax and Avro Lancaster, and the deployment of Gee. The two, combined, would offer the accuracy and weight of bombs that Lindemann's calculations called for. Efforts to test and deploy Gee became a high priority, and the Chain Executive Committee was set up under the chairmanship of Robert Renwick in October 1941 to site a series of Gee stations. Gee was not the only solution being developed; it was soon joined by H2S radars and the Oboe system.

As the initial availability of the Gee devices would be limited, the idea of the pathfinder force was adopted. This concept had originally been developed by the Luftwaffe for their early night raids against England. Lacking enough radio sets and the widespread training to place their radio navigation systems on all their aircraft, they collected what they had into the single group, KG100. KG100 would then use their equipment to drop flares, which acted as an aiming point for following bombers.

Eager to test the Gee system, prototype sets were used on target indicator aircraft well before the production sets were available in the number required for large raids. On 15 May 1941, such a set provided an accurate fix at a range of 400 miles at an altitude of 10,000 feet. The first full transmitter chain was completed in July 1941, but in testing over the North Sea, the sets proved to be unreliable. This was traced to the power supplies and tubes, and corrections were designed and proved that summer.

On the night of 11/12 August, two Gee-equipped aircraft bombed using Gee coordinates only and delivered "uncanny accuracy".[4] However, on the next night on a raid over Hanover, a Gee-equipped Vickers Wellington was lost. The Gee set did not contain self-destruct systems, and it might have fallen into German hands.[7] Operational testing was immediately suspended.[4]

R. V. Jones responded by starting a disinformation campaign to hide the existence of the system. First, the use of the codename 'Gee' in communications traffic was dropped, and false communications were sent referring to a fictitious system called 'Jay'; it was hoped the similarity would cause confusion. A double agent in the Double Cross system reported to German Intelligence a fictional story of hearing a couple of RAF personnel talking carelessly in a hotel about Jay, and one dismissing it as it was "just a copy" of the German Knickebein system. Jones felt this would flatter the Germans, who might consider the information more reliable as a result. Extra antennae were added to the Gee transmitters to radiate false, unsynchronized signals. Finally, false Knickebein signals were transmitted over Germany.[8] Jones noted all this appealed to his penchant for practical joking.

In spite of these efforts, Jones initially calculated only 3 months would be needed before the Germans would be able to jam the system. As it turns out, jamming was not encountered until five months into the campaign, and it was much longer before it became a serious concern.[9]

Even with limited testing, Gee proved itself to be easy to use and more than accurate enough for its tasks. On 18 August 1941, Bomber Command ordered Gee into production at Dynatron and Cossor, with the first mass-produced sets expected to arrive in May 1942. In the meantime, a separate order for 300 hand-made sets was placed for delivery on 1 January 1942[10] which was later pushed back to February. Overall, 60,000 Gee sets were manufactured during World War II, used by the RAF, USAAF, and Royal Navy.[11]

The first operational mission using Gee took place on the night of 8/9 March 1942, when a force of about 200 aircraft attacked Essen. It was installed on a Wellington of No. 115 Squadron from RAF Watton captained by Pilot Officer Jack Foster, who later said, "targets were found and bombed as never before".[12]Krupp, the principal target, escaped bombing, but bombs did hit the southern areas of the city. In total, 33% of the aircraft reached the target area, an enormous advance over earlier results.[13]

The first completely successful Gee-led attack was carried out on 13/14 March 1942 against Cologne. The leading crews successfully illuminated the target with flares and incendiaries and the bombing was generally accurate. Bomber Command calculated that this attack was five times more effective than the earlier raid on the city. The success of Gee led to a change in policy, selecting 60 German cities within Gee range for mass bombing using 1,600–1,800 tons of bombs per city.[13]

To provide coverage of the entire UK, three Gee chains were constructed under the direction of Edward Fennessy.[14] The original chain started continuous operation on 22 June 1942, followed by a chain in Scotland later that year, and the southwest chain in 1943. Even as German jamming efforts took hold, Gee remained entirely useful as a short-range navigation system over the UK. Only 1.2% of Gee-equipped aircraft failed to return to their base, as opposed to 3.5% of those without it.[15] Gee was considered so important that a nonworking Gee set would ground an aircraft.[16]

One illustration of Gee's routine employment by Bomber Command in navigation tasks was its use (albeit a limited one) in Operation Chastise (commonly known as the "Dam Buster Raid") in May 1943. In his memoir, Enemy Coast Ahead,[17]Guy Gibson, the leader of the raid, briefly mentions his navigator, F/O 'Terry' Taerum, RCAF, employing what Gibson calls Taerum's "G Box" to determine groundspeed while flying very low at night over the North Sea from Britain to Holland, en route to Germany.

Low-level photo of a light mobile Gee station operating in a field near Roermond, Holland. These forward stations provided Gee coverage deeper into Germany, as well as strong signals for aircraft returning to bases in Western Europe.

The first serious jamming was encountered on the night of 4/5 August 1942. This grew in strength as the bombers approached their target at Essen, and the signals became unusable at 10 to 20 miles (16–32 km) from the target. The newly formed southern chain was not yet known to the Germans and continued to be useful. On 3/4 December, a fix was made from this chain over Turin in Italy, at a range of 730 miles. This remained the operational record for Gee, bested only by a freak reception over Gibraltar at a range of 1,000 miles.[16]

Counter-jamming efforts had already been considered, and resulted in the Gee Mk. II. This replaced the original receiver with a new model where the oscillators could be easily removed and swapped out to provide a range of operational frequencies. These included the original 20– to 30-MHz band, as well as new bands at 40–50, 50–70, and 70–90 MHz. The navigator could replace these in flight, allowing reception from any active chain. Gee Mk. II went into operation in February 1943, at which point it had also been selected by the US 8th Air Force.[18]

On 23 April 1942, the go-ahead was given to develop mobile stations for Gee in preparation for the invasion of Europe. This would not only extend the range of the system eastward, but also allow stations to move and suddenly appear elsewhere if jamming became an issue. The first of an eventual three such mobile chains was formed up on 22 November 1943. This was put into operation on 1 May 1944 at Foggia in Italy, and was used operationally for the first time on 24 May. Other units were sent into France soon after D-Day. The mobile units in France and Germany were later replaced by fixed stations, the "heavies".[19]

After the end of the war in Europe, Britain planned to send Lancasters to the Japanese theatre as part of Tiger Force and to use Gee for the passage of flights to Asia. Preparations began for Gee transmitters in Nablus (in Palestine) guiding the flights across the Middle East, but the surrender of Japan removed the need for this chain. This work was being carried out by MEDME, Cairo, under Air Vice Marshall Aitken.

German bombers also used the Gee system for attacks on the UK; captured Gee receivers provided the electronics.[20]

Later in the war, Bomber Command wanted to deploy a new navigation system not for location fixing, but to mark a single spot in the air. This location would be used to drop bombs or target indicators for strikes by other bombers. The Oboe system provided this already; Oboe sent an interrogation signal from stations in the UK, "reflected" them from transceivers on the aircraft, and timed the difference between the two signals using equipment similar to Gee. However, Oboe had the major limitation that it could only guide a single aircraft at a time and took about 10 minutes to guide a single aircraft to its target. A system able to guide more aircraft at once would be a dramatic improvement.

The result was a new version of the same basic Oboe concept, but reversed so that it was driven by the aircraft and reflected from ground-based transceivers. This would require equipment on the aircraft that could receive and measure the time difference between two signals. The reuse of the existing Gee equipment for this purpose was obvious. The new Gee-H system only required a single modification, the addition of a new transmitter that would send signals out for reflection from ground-based transceivers. With this transmitter turned off, the system returned to being a normal Gee unit. This allowed it to be used in Gee-H mode during attacks, and then Gee mode for navigation back to their home airfields.

Gee was of such great utility that the hurried deployments during the war were rationalized as the basis for an ongoing and growing navigational system. The result was a set of four chains, South Western, Southern, Scottish, and Northern, which have continuous coverage over most of the UK out to the northeastern corner of Scotland. These were joined by a further two chains in France, and a single chain in the UK occupation zone in northern Germany.[21]

A single leg of a Gee chain lies along the "baseline" from stations A to B. At any point between these stations, a receiver will measure a difference in timing of the two pulses. This same delay will occur in many other locations along a hyperbolic curve. A navigational chart showing a sample of these curves produces a graph like this image.

Hyperbolic navigation systems can be divided into two main classes: those that calculate the time difference between two radio pulses, and those that compare the phase difference between two continuous signals. Here, only the pulse method is considered.

Consider two radio transmitters located at a distance of 300 km from each other, which means the radio signal from one will take 1 millisecond to reach the other. One of these stations is equipped with an electronic clock that periodically sends out a trigger signal. When the signal is sent, this station, the "master", sends out its transmission. A millisecond later, that signal arrives at the second station, the "slave". This station is equipped with a receiver, and when it sees the signal from the master arrive, it triggers its own transmitter. This ensures that the master and slave send out signals precisely 1 ms apart, without the slave needing to have an accurate timer of its own. In practice, a fixed time is added to account for delays in the electronics.[6]

A receiver listening for these signals and displaying them on an oscilloscope sees a series of blips on the display. By measuring the distance between them, the delay between the two signals can be calculated. For instance, a receiver might measure the distance between the two blips to represent a delay of 0.5 ms. This implies that the difference in the distance to the two stations is 150 km. In this case, an infinite number of locations exist where that delay could be measured – 75 km from one station and 225 from the other, or 150 km from one and 300 from the other, and so on.

When plotted on a chart, the collection of possible locations for any given time difference forms a hyperbolic curve. The collection of curves for all possible measured delays forms a set of curved radiating lines, centred on the line between the two stations, known as the "baseline".[6] To take a fix, the receiver takes two measurements based on two different stations. The intersections of the two sets of curves normally results in two possible locations. Using some other form of navigation, dead reckoning for instance, one can eliminate one of these possible positions, thus providing an exact fix.[1]

Instead of using two separate pairs of stations, the system can be simplified by having a single master and two slaves located some distance away from each other so their patterns overlap. A collection of such stations is known as a "chain".[1]

Gee chains used an arrangement with one master and (typically) three slaves. The signals from the master and either of the two slaves would be active at any time, with the third station and a chain monitor station providing backup and signal quality measurements. The transmitters had a power output around 300 kW and operated in four frequency bands between 20 and 85 MHz.[6]

The Gee signal for any given chain consisted of a series of pulses of radio signal with a roughly inverted-parabolic envelope about 6 microseconds in duration.[22] The master sent a single pulse, referred to as "A", followed 2 ms later by a double pulse, "D". These were used to identify the start and end of the broadcast cycle. The first slave station sent a single pulse 1 ms after the master's single pulse, labeled "B", and the second slave sent a single pulse 1 ms after the master's double pulse, labeled "C".[6] The whole sequence repeated on a 4 ms cycle, with the pattern "ABD... ACD... ABD..."[c]

The triggering of the A pulses was timed at 150 kHz by a stable local oscillator at the master station,[23] but the timing was sometimes deliberately changed. The time for ten cycles of this 150 kHz oscillation, 66.66 μs was called a Gee unit and corresponded to a range difference of 12.4 miles (20.0 km).[23]

In Gee, all of the signals in a given chain were sent on the same frequency, and there was no attempt to modify the pattern of the individual envelopes from the slaves to allow identification. This meant that it would normally be possible to confuse the "ABD" for the "ACD", as they would look identical on the screen. In order to resolve these problems, the master station periodically sent a third pulse, "A prime" or the "ghost A". These were folded into the signal only on the "ACD" cycle, allowing the operator to visually identify the B and C portions of the cycle.[24]

On board the aircraft, the signals from the three stations were received and sent to the 'A-type' display. A local oscillator of much less complexity than the one at the master station was used to trigger the display sweep. When first activated it would be unlikely to have exactly the same timing as the master station, so when it was first turned on, the operator would see a repeating pattern of blips travelling rapidly across the screen. A control was used to tune the local oscillator frequency until the blips on the display stopped moving, which meant the local and master oscillators now had the same timing.[24]

The sweep of the beam across the display was initially set to 1/10 the basic frequency of the local oscillator, so approximately five complete signal chains would be seen on the display. Once the operator had tuned the frequency so the image was frozen, a switch was thrown to increase the beam speed 10 times. This reduced the amount of the signal display on the display to one complete 2 ms A...D cycle.[24]

At this point a delay was used to move the entire pattern left or right until the A signal was on the left side of the display. A fine timing control could then be used to adjust the speed of the time base until the cycle exactly filled the screen with the A signal on the left and the D signal on the right.[24]

The Gee display also included a second control unit to create a second line on the lower portion of the screen. This was set up in the same fashion, but this time the navigator would delay the signal so that the AA' signal was placed on the extreme left. Once the initial setup was completed, the upper line displayed the "ABD" signal, and the lower the "ACD". This allowed the two delays to be read simultaneously against the scale, and then looked up on the navigational chart.[24]

Signals from different chains were closely spaced in frequency, close enough that the wide-band R1355 receiver would often tune in more than one chain at a time. For station identification, the A' signals were only sent periodically. After the display was stabilized so the pulse trains were appearing in a single location on the screen, the A' pulses could be seen blinking on and off with a set pattern (thus "ghosting" on the display). This allowed the operator to determine the identity of the master signal, and thus select the chain they wanted to use by positioning its associated "ABD" signal on the left.[24]

At long ranges the hyperbolic lines approximate straight lines radiating from the center of the baseline. When two such signals from a single chain are considered, the resulting pattern of lines becomes increasingly parallel as the baseline distance becomes smaller in comparison to the range. Thus at short distances the lines cross at angles close to 90 degrees, and this angle steadily reduces with range. As the accuracy of the fix depends on the crossing angle, all hyperbolic navigation systems grow increasingly inaccurate with increasing range.[25]

When examining the signal on the expanded display, timing was based on 1/10 of a Gee unit, or 6.66 μs. This corresponds to a distance of 1.24 miles (2,000 m). It was assumed that an operator under good conditions could measure the peaks of the pulse envelope within 1/10 of a calibration mark, or 0.124 miles (200 m). This is the basic accuracy of the Gee system, at least at shorter ranges and at locations near the center of the baselines where the hyperbolic lines were close to perpendicular. In practice, the accuracy was a function of range from the transmitters, varying roughly with the square of distance.[26] At short ranges accuracies of 165 yards (151 m) were reported, while at long range over Germany it was quoted to about 1 mile (1.6 km).[24]

The airborne side of the Gee Mk. II system consisted of two portions, the R1355 radio receiver, and the Indicator Unit Type 62 (or 62A) oscilloscope. The two were attached together by two thick cables, one of which carried the video signal, and the second carried the power to the Indicator, the power supply being built into the receiver to save space on the display side.[27] A "tropicalized" version of the system was also produced, with the R3645 receiver and Indicator Unit Type 299, which moved the later's power supply into the display unit.

The R1355 was designed so as to allow the Radio Frequency Unit (RFU) to be easily swapped over in flight. This allowed the navigator to select different Gee Chains, the change-over operation taking only a minute or so. A change of RFU could also be used to avoid jamming, as the Germans would not know which Chains were being actively used.

In the case of Gee-H, the use of the system was changed only slightly. Instead of the sweep being timed by the local oscillator in the display unit, the trigger signal was instead sent from an onboard timer. The signal was also amplified and sent out to interrogate the distant ground stations, whose response signals were received on the existing Gee receiver. In theory this could be used to calculate a fix in exactly the same fashion as with Gee, using different charts. However, navigating to a target using such a system would be complex, similar to using dead reckoning, with periodic fixes needed to update the windage calculations.

Instead, Gee-H was used in a fashion similar to the earlier Oboe system. The navigator would first pick a station to use as the "cat" signal, using it as the main navigation beacon. The range from the cat station to the target was measured, and then the signal delay that would be seen at that range was calculated. A timer in the unit was set to this delay, producing a separate A-like blip on the display at that fixed point. The received pulse from the "cat" station would also be displayed on the same trace. By instructing the pilot to turn left or right, the navigator would guide the bomber until the two traces were precisely overlapped, meaning that the bomber was flying a precise distance from the station, along a circular arc that would take them over the aim point. The signal received from the second station, "mouse", was likewise set up for display on the lower trace, but in this case the distance would continue to change as the aircraft flew along the arc of the "cat" station. When this signal overlapped the pre-set range from "mouse", the payload was dropped.

Using this method of operation greatly reduced the workload for the navigator. For much of the mission he simply had to keep the blips on the upper trace aligned on the display, and then periodically watch the lower blips for timing. Additionally, due to the measurements always being measured as direct lines from the station, as opposed to hyperbolic curves, the accuracy fell off linearly instead of with the square of distance. Gee-H could this guide the bomber to within 120 yards over Germany, a dramatic improvement over Gee's approximately 1 mile accuracy at the same distance.

Unlike the German beam systems where the bombers flew to their targets along the beam, Gee pulses were radiated in all directions so, if detected, they would not reveal the bombers' destinations.[1] As the system was passive, unlike H2S, there were no return signals which could give away the bombers' positions to night fighters. Additionally, this meant all of the aircraft could use the system at the same time.

Gee was highly susceptible to jamming since all the Germans had to do was radiate spurious pulses that made it impossible to determine which was a real signal from the slave and which was being broadcast from a jammer. This could be easily arranged by locating another slave station in France or the Netherlands and modifying its delay and signal strength for its signals to appear to be similar to those from one of the stations in the UK. This worked only over Germany; when aircraft flying over the UK the signal would appear too weak. Using conventional radio receivers and loop antennas for direction finding, radio operators could determine which of the signals was false. However even if jammed over enemy territory, Gee had the extremely useful advantage of providing a reliable navigational 'fix' once homeward aircraft were over the North Sea on return from operations, making it easier for returning bombers to find their airfields with a resultant reduction in losses due to accidents.

Test work, including timing and signal strength, was undertaken at several places for much of 1941, with the 4 transmitter stations at Daventry (master), Ventnor, Stenigot and Clee Hill (slaves), and the monitoring station at Great Bromley running as an experimental chain from July.

The Eastern chain became operational (as opposed to conducting tests) from March 1942, and was used in "Bomber" Harris's major and unprecedentedly successful raids on Lubeck and Cologne that spring, Its HQ and monitoring station, initially at Great Bromley, moved on to Barkway that November. Wing Commander Phillips, assisted by Squadron Leader Allerston and the scientific side Edward Fennessey, were then in charge.
(AVIA 7/1251, AIR 29/147 & other National Archive Files; Air Ministry Maps of Gt Bromley and Barkway sites at RAF Museum; J P Foynes "AMES 24:The Pylons at Great Bromley").
22 June 1942. Virginia[28] 48.75 MHz[28]

Post-World War II the RAF re-sited two of the three wartime Gee chains in England. Eastern and South Western chains (four stations each) and Southern chain of three stations. The Southern chain became a four station London chain and Eastern chain became a Midland chain. This was planned for 1948.[31]

There were several stations during the 1955–59 period that appeared to be more of a deception than really operational. They were 550 SU at Fort Spijkerboor outside of Purmerend, Holland; 889 SU at Eckernförde in North Germany; and 330 SU outside of Ingolstadt in Bavaria, Germany. These stations were rarely if ever operation in the late 1950s. 330Su was an amalgamation of 3 units 330. 259 and 953 signals units and was in continuous operation at Ingolstadt from May 1958 to September 1961. The other 2 units forming this chain were at Oberkirchen and Schleswig. All three had particular attractions: Ingolstadt had access to the main US Army PX in Munich. Oberkirchen was close to the NAAFI winter sports center and Schleswig was on the dunes by the nude bathing area.

^Different sources record the name as GEE or Gee. The naming supposedly comes from "Grid". "Gee" is used in Dippy's publications. See Dippy 1946. The Air Ministry sometimes referred to it as Gee-7000. or Gee-7000, see "An Introduction Survey of Radar, Part II", Air Ministry, June 1946

^Although it is not mentioned in available sources, some method of telling the two signals apart would be needed. The solution used on Gee, periodically turning one of them on and off to cause it to oscillate on the display, would work here as well.

^Haigh and Blanchard differ in their description of the pulse chains. Blanchard describes the ABD signal described in this article. Haigh suggests there was no D signal, and the pattern was "AB...AA'C...AB...". Brown suggests the double-pulse was sent from the third station in the chain, although how this would remain synchronized with the master is unexplained. These may be due to operational changes during the war.